Zinc is an ion of growing importance in many field of biology and medicine. In particular, recent work has demonstrated the excitotoxic role(s) of zinc in the brain, [1-3] as well as its potential role as a signaling ion in the brain [4] which recent evidence suggests participates in long term potentiation [5]. Elsewhere in the body, zinc seems to play a role in the immune response [6], and is a prevalent constituent of semen, as well as an essential cofactor in many enzymes [7] and the ubiquitous “zinc fingers” of transcription factors [8]. The role of zinc in apoptosis is the subject of controversy [9], and there is no consensus as to how zinc is distributed in the body, allocated amongst its many role(s), or how these processes are regulated. The availability of selective, sensitive, quantifiable fluorescent calcium indicators beginning with Quin-2, Fura-2, and Indo-1 [10] has revolutionized our understanding of calcium, and if analogous indicators were available for zinc, perhaps comparable progress could be made. Despite substantial effort [9, 11-16], it is only recently that fluorescent zinc indicators have been made which offer adequate selectivity over potential interferents such as Ca and Mg; reliable quantitation through intensity ratios, anisotropy, or fluorescence lifetime; and useful sensitivity. In particular, the recently introduced FuraZin-1 and Newport Green DCF from Molecular Probes offer selectivity (Thompson, et al., J. Neuroscience Methods (2002), hereby incorporated by reference), micromolar sensitivity, and quantitation by excitation intensity ratio (FuraZin) and fluorescence lifetime (Newport Green).
However, substantial recent evidence suggests that, as in the case of calcium elucidation of the biology of zinc will require in many cases significantly better sensitivity than the above indicators offer. In particular, release of zinc into the ventrical of rabbit brain following transient global ischemia or blunt force trauma yields peak levels in the nanomolar range, against a background of less than five nanomolar (Frederickson, et al., in preparation). The affinity of the NR2A subunit of AMPA receptor for Zn(II) has been measured in vitro to be 20 nM[17], suggesting that it responds to zinc levels in this regime. Ordinarily the free Zn(II) concentration in serum is one nanomolar or less, based on measurements and calculations incorporating the affinities of the two principal Zn buffers, α2-macroglobulin and serum albumin [18]. Recent work suggests that free Zn(II) concentrations may be very low in bacterial cells [19]. While other recent results (Thompson, et al., submitted) [20] indicate that the stimulus-induced release of zinc in hippocampus is in the micromolar range, the lower range indicated by the above measurements suggests that it will be necessary to measure lower concentrations, particularly inside cells.